Makefile: Improve compiler header dependency check

The Makefile enables CHECK_HEADER_DEPENDENCIES when the compiler supports generating header dependencies. Make the check use the same flags as the invocation to avoid a false positive when user-configured compiler flags contain incompatible options. For example, without this patch, trying to build universal binaries on a Mac using CFLAGS='-arch i386 -arch x86_64' produces: gcc-4.2: -E, -S, -save-temps and -M options are not allowed with multiple -arch flags While at it, remove "sh -c" in the command passed to $(shell); at this point in the Makefile, SHELL has already been set to a sensible shell and it is better not to override that. Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>